This application claims the priority of German Patent Document No. 100 46 692.3, filed on Sep. 21, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a device for evaporating a liquid having (1) a plurality of chambers for carrying out a solid-catalyzed reaction, and (2) a common evaporation unit for evaporating liquid starting materials. The evaporation unit is in thermally conductive contact with the plurality of chambers.
Solid-catalyzed processes and the devices required for these processes are in increasingly widespread use in engineering.
One example is the use of fuel cells for supplying energy to homes and motor vehicles (Ullmann's Encyclopedia der technischen Chemie, Volume 12, pp. 113–136, Verlag Chemie, Weinheim 1976). Another example is that of galvanic cells to reduce carbon monoxide emissions.
In installations of this type, a simple, compact design which takes up little space while at the same time needing small quantities of catalyst material, which is often expensive, and a high conversion rate are important.
This is illustrated, by way of example, with reference to hydrocarbon reforming reactors. For example, to generate hydrogen from hydrocarbons (for example methanol) and water, it is proposed in EP 0 906 890 A1 to press the reaction starting materials through layers which are stacked on top of one another and contain catalyst material. Consequently, the flow resistances, which often cause problems when supplying and discharging liquid starting materials and products, are of no importance.
DE 196 54 361 A1 likewise describes a stacked reactor in which the individual layers which are stacked on top of one another are connected via reaction channels which are in fluid communication. Unpublished document DE 199 04 398 discloses a stacked reactor comprising discs which are arranged flush above one another and are provided with a single aperture and contain catalyst material. Liquid reaction starting materials can be metered onto the individual discs via the reaction channel which is formed from the apertures and is provided with a metering lance. Stacked reactors of this type are versatile on account of their modular, compact structure.
Generally, hydrocarbon reforming takes place in two or more stages. In a first stage, only partial conversion of the hydrocarbon is aimed for in a first reactor. In a second stage, the reaction mixture containing starting materials and products is passed to a second reactor, which is optimized for the remaining conversion, as described, for example, in EP 0 687 648 A1. EP 0 217 532 B1 has disclosed a further reactor which comprises two reaction zones one above the other.
However, the reactors which have been disclosed hitherto have the problem that, when using one or more reaction starting materials which are in the liquid state, the reactor cannot be operated at its rated capacity, since the liquid cannot be directly applied to the catalytic layer by the conventional metering methods, for example nozzles.
Furthermore, there are problems when starting the reactor, since the surface area which comes into contact with the liquid is insufficient to evaporate the liquid which is, for example, sprayed on. This leads to the reactor not commencing operation even after the catalytic reaction has started, on account of the quantity of liquid suddenly being too great, and even, in extreme cases, to the catalytic reaction failing altogether. Therefore, in many solid-catalyzed reactions, it is often desirable for a reaction starting material to be supplied to a catalyst layer already substantially in vapor form.
DE 199 07 665 A1 discloses a device with a stacked reactor in which an evaporator is heated by heat exchange with hot synthesis gas. In one proposed embodiment, the evaporator, at the end side, butts against the flat side of the outermost reactor disc and is heated in a countercurrent by the hot synthesis gas formed in the reactor. For this purpose, the synthesis gas is collected, discharged from the reactor, and guided past the outer wall of the evaporator.
The present invention is based on an object of providing a device for carrying out a solid-catalyzed reaction which allows uniform, continuous evaporation of a starting material in such a manner that subsequent uniform distribution of a starting material over a plurality of catalyst layers is achieved.
According to the present invention, a device for carrying out a solid-catalyzed reaction includes a plurality of chambers for carrying out the solid-catalyzed reaction and a common evaporation unit for evaporating liquid starting materials. The evaporation unit is in thermally conductive contact with a plurality of chambers. An area of the evaporation unit in which the evaporation substantially takes place is at least partially surrounded by the plurality of chambers.
The device according to the present invention ensures that the evaporation of a starting material takes place continuously and completely in the entire evaporation unit, so that then, by a suitable distribution system, starting-material vapor is passed homogeneously and uniformly into reaction chambers with catalyst layers and can be fed through or onto the catalyst layers. Moreover, the use of evaporated starting material significantly increases the reaction rate of the reaction in question on the catalyst layer.
In the text which follows, the term “catalyst layer” refers to a layer of any desired geometric shape, structure, and composition which comprises material which reduces the activation energy of a previously selected reaction which takes place on the surface of or inside this layer. This material may also be described by the term “catalyst”.
In the text which follows, the term “evaporation unit” is understood as meaning any suitable device at or in which, through the supply of heat, a substance which is in the liquid state can be converted into the vapor phase.
The evaporation unit takes the heat required to evaporate starting materials from the exothermic catalytic reaction which takes place at the catalyst layers. The chambers which contain catalyst layers may also be referred to as modules. The reaction is commenced by methods which are known per se, and the heat from this reaction then feeds the evaporator via the thermally conductive connection. In this case, the heat is introduced substantially directly through contact with the hot reactor zone. The evaporation unit is situated in the region of the hot reactor zone, and the evaporation therefore takes place in this region.
The thermally conductive connection is effected in such a manner that the resulting temperature distribution in the evaporation unit is ideal for operation of the device, for example for a stacked reactor of modular construction. In the case of a modular reactor, it should preferably be ensured that the temperature is identical in all the modules.
However, it is also possible to establish a temperature gradient between the modules. If more heat is to be extracted from one module, the thermal coupling to the evaporator may be greater in that module than in other modules, in which, for example, less heat is to be extracted.
Furthermore, it is possible for only one starting material to be evaporated beforehand, while another starting material is fed directly onto the catalyst layer, separately or together with the first evaporated starting material, via other means. In general, however, the entire reaction mixture is evaporated in advance. In specific cases, however, this will be dependent on the reaction which is to be carried out.
In a preferred configuration, the evaporation unit comprises a plurality of channels as shown in FIG. 3 which are arranged parallel to one another, since it is expedient to supply a plurality of smaller channels of the evaporation unit with heat than to supply a large central evaporation unit. The surface area of the evaporation unit is increased, which leads to improved heat transfer between catalyst layer and evaporation unit. The branching also readily allows multiple contact with the catalyst layers, for example by an arrangement in which one or more bores are guided onto each catalyst layer, and the evaporation unit is then arranged in these bores. However, this too is dependent on the reactor design and the reaction which is to be carried out.
The preferred reactor can be started quickly and efficiently and can therefore be operated with its rated capacity within a very short time.
The spatial arrangement of the evaporation unit on the modules of a reactor can be as desired. For example, it is conceivable to use arrangements on the outer edge of the modules or of the entire device.
The evaporation unit is preferably rigidly connected to the chambers and/or the catalyst layer, since this allows thermally conductive connection with the hot reactor zone without great heat losses.
Depending on the reaction and the heat which is liberated in the reaction, it may also be advantageous to make the connection movable, so that extreme stresses caused by thermal gradients between catalyst layer and evaporation unit, which could cause irreversible damage to the evaporation unit, are avoided.
The device according to the present invention is suitable for all known solid-catalyzed processes, in particular, however, for reforming hydrocarbons. The term hydrocarbons is also understood as meaning compounds such as alcohols, aldehydes, ketones, ethers, and the like. The device can be used to produce hydrogen for fuel cells.
The result is a procedure which is efficient and simple to design, so that the starting material, which is evaporated continuously and completely, can be fed to the catalyst layers via suitable distribution systems. The uniform evaporation allows continuous and controllable supply of evaporated starting material with a uniform distribution to all the modules of a reactor, which is therefore ready for operation more quickly.
It will be understood that the features which have been referred to above and those which are still to be explained below can be used not only in the combination indicated in each case but also in other combinations or on their own without departing from the scope of the present invention.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the present invention when considered in conjunction with the accompanying drawing.